hw/arm/aspeed: Move AspeedMachineState definition to common header for reuse

hw/arm/aspeed: Move AspeedMachineState definition to common header for reuse

Aspeed machines will be moved into split C files for better
modularization and future maintenance.

To allow all machine implementations to reuse the same
AspeedMachineState structure, the struct definition is moved
from aspeed.c to the shared header aspeed.h.

This change centralizes the common state structure used across
all Aspeed machine models, reduces redundancy, and simplifies
future refactoring work for new machines.

No functional changes.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20251104031325.146374-2-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>